42 Canine lens luxation
Dogs are presented with a painful eye when suffering from acute anterior lens luxation, the most frequently encountered form in dogs. Normally the owner notices a sudden onset of blepharospasm and increased lacrimation and the eye also appears both red and cloudy. Vision is frequently reduced but owners might not be aware of this if the other eye is normal. Terriers are most commonly affected and young adults, 4–6 years, are most at risk. Affected animals can be depressed and unwilling to eat due to the pain from the condition. Most owners realize that there is something wrong and bring their pet to the surgery quickly, but unfortunately this is not always the case. Breeders will be aware of the condition and as such are more likely to present affected dogs as a matter or urgency.
In most cases there is no previous history of ocular disease, or relevant systemic disease. The dog might have been out playing, such that the owner suspects a traumatic incident, but this is rarely witnessed (and usually has not occurred!). Sometimes the owner will have been aware that the eye was a bit red and watery for a few days before it suddenly became acutely worse.
On general clinical examination the patient might be subdued and unwilling for the face to be touched – especially on the affected side. Mild pyrexia can be present due to the pain of the condition but otherwise the examination is unremarkable.
Ophthalmic examination is likely to reveal reduced vision on the affected side. A moderate serous ocular discharge might be present, along with blepharospasm and photophobia. It might be difficult to open the eye for examination, especially in moderately enophthalmic breeds such as the miniature English bull terrier. Application of topical anaesthetic drops might assist the examination. Corneal oedema is frequently present, sometimes affecting the whole of the eye (when severe secondary glaucoma has set in) or just in the subcentral area, with a cloudy patch visible. Episcleral congestion will be present. Ulceration is not normally a feature unless the patient has self-traumatized.
The lens can be visualized in the anterior chamber between the back of the cornea and the front of the iris (Figure 42.1). By viewing the patient from the side it should be possible to see the iris bowing back away from the cornea and the structure of the lens should be apparent. The size of the pupil is variable (if visible!) but is normally mid sized or dilated, and often fixed or poorly responsive to light. If fundus examination is possible it could be normal, or optic disc cupping might be noted (discussed further in the cases on glaucoma). Intraocular pressure measurement should be undertaken if possible, since this will have an important effect on the prognosis and management of the cases.
Figure 42.1 Anterior lens luxation in a 4-year-old Jack Russell terrier. The lens can be seen sitting in the anterior chamber and a subcentral patch of corneal oedema is present where the lens is resting against the corneal endothelium.
Positive diagnostic tests which can be performed in practice are simple – if there is a positive dazzle reflex in the affected eye, and a consensual pupillary light response in the unaffected eye, i.e. on shining a light into the affected eye the fellow pupil constricts, these both indicate that vision might be preserved with rapid referral for lendectomy. If neither test is positive, then the prognosis for vision is very poor, and any emergency surgery is only likely to salvage the globe.
The fellow eye should be very carefully evaluated since primary lens luxation is a bilateral condition, although it is rare for both lenses to luxate simultaneously. The contralateral lens is frequently subluxated – this can be appreciated as iris wobble (iridodonesis) or lens wobble (phacodonesis) as the eye moves. The anterior chamber might be deep or shallow, depending on where the lens has slipped, and strands of vitreous are frequently visible in the anterior chamber. Pupillary dilation with tropicamide might allow the edge of the lens to be visualized – normally it slips slightly ventrolaterally, and stretched lens zonules can sometimes be noted (Figure 42.2). The worse affected eye (with the full anterior luxation) should not be dilated with tropicamide since this could both exacerbate any glaucoma and allow the lens to fall into the vitreous, making surgical retrieval much more difficult.
Figure 42.2 Subluxated lens in a 5-year-old miniature English bull terrier. An aphakic crescent can be seen dorsomedially with stretched lens zonules just visible in this area. The pupil has been fully dilated to appreciate this lens instability although iridodonesis was present clinically.
Some patients will not have an anterior but a posterior lens luxation. Typically with these the owner reports the eye was very sore and cloudy, but then became more comfortable, although it still did not look normal. In such cases the subcentral patch of corneal oedema will still be present but the lens is not visible in the anterior chamber (Figure 42.3). The iris normally wobbles (iridodonesis) on globe movement and the anterior chamber is often deep. By looking though the pupil from above, the lens can normally be seen lying in the ventral posterior segment. The different subtypes of lens luxation are shown in Figure 42.4.
Figure 42.3 Posterior lens luxation in an elderly German shepherd dog. There is a patch of corneal oedema but no lens in the anterior chamber or pupil – with careful examination it will be found ventrally in the vitreous.